feat(android): show active-thread context-window usage near composer #49

Request

Add a compact context-window display to Android, similar to Dash: used tokens / capacity and percentage, with tap-through details.

Triage

Reuse normalized provider context usage and shared derivation already used by web. Do not confuse cumulative billing tokens or account limits with current context occupancy. Upstream related data gaps: pingdotgg#26476, pingdotgg#16148, pingdotgg#8388; this issue is mobile presentation and honest unavailable states, not fabricating missing telemetry.

Acceptance

• Compact, accessible composer-adjacent context display with details.
• Known used/max shows correct percentage; unknown capacity is clearly unavailable.
• Updates after turns/compaction and follows thread/model/environment changes without stale values.
• No hardcoded guessed model capacities; no per-frame animation.
• Focused tests for known, missing, zero and over-capacity data.

Fork-local implementation first; native acceptance distinct from static tests.